METHOD FOR PREPARING GRAPHENE RIBBONS

Abstract

Disclosed is a method for fabricating graphene ribbons, comprising: preparing a graphitic material comprising stacked graphene helices; and cutting the graphitic material in a short form by applying energy to the graphitic material; and simultaneously or afterward, decomposing the graphitic material into short graphene ribbons. This method provides a mass production route to graphene ribbons.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

The present disclosure relates to subject matter contained in priority Korean Application No. 10-2000-0082512, filed on Aug. 22, 2008, which is herein expressly incorporated by reference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method for preparing graphene ribbons, and particularly, to a method for fabrication of graphene in the form of a ribbon.

2. Background of the Invention

Graphene, a single layer trigonal carbon honeycomb with a thickness of about 4 Å (refer to FIGS. 1(A) and 1(B)), has enormous potential due to its outstanding physical properties compared even to single-wall carbon nanotubes. It is the basic unit of C₆₀, multi-walled carbon nanotubes (MW CNTs), and graphite.

Due to the weak van der waals attraction between graphene layers, the two-dimensional material is obtainable when the forces between graphene planes are disrupted. Micromechanical cleavage is the most assured method for fabricating graphene, but the yield is too small. The yield of pure graphene in a chemical route, which has been proposed as a mass production method, is also as low as around 0.5%. Graphene formed on a metal substrate by chemical vapour deposition (CVD) methods produce mostly multiple graphene layers.

On the other hand, there have been efforts to prepare short carbon nanotubes by cutting multi-walled carbon nanotubes (known as a non-crystalline turbostratic structure, refer to FIG. 2) with a mechanical method such as ball milling or a chemical method (refer to L. Chen et al., [Materials Letter 60 (2006) 241-244], N. Pierard et al., [Chemical Physics Letters 335 (2001) 1-8], Z. Konya et al., [Carbon 42 (2004) 2001-2008], and Z. Gu et al., [Nano Letter 2 (2002) 1009]). However, graphene could not be obtained by such efforts.

SUMMARY OF THE INVENTION

Therefore, an object of the present invention is to provide a mass production route to graphene ribbons, thus opening up industrial applications utilising the large scale i.e., tonnes a year, of this innovative carbon.

To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described herein, there is provided a method for preparing graphene ribbons, crumbling the graphitic materials composed of long graphene helices (˜μm in length) (refer to FIG. 3) into short graphene ribbons (˜50 nm in length) by applying energy (refer to FIGS. 4(A)-(D)). The graphene ribbon based materials are stacked of AA′ (refer to FIGS. 3 and 5) or turbostratic (refer to FIG. 2) of which an interlayer bond force is weaker than that of an AB (refer to FIGS. 1(A) and 1(B)). This development provides a mass production route to graphene ribbons, thus opening up industrial applications utilising the large scale i.e., tonnes a year, of this innovative carbon.

The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and together with the description serve to explain the principles of the invention.

In the drawings:

FIG. 1(A) is a schematic diagram of graphite with an AB stacked structure, and FIG. 1(B) is a planar view of AB graphite showing the feature of the AB stacking of graphene layers;

FIG. 2 is a planar view of turbostratic graphite showing the feature of the disordered turbostratic stacking of graphene layers;

FIG. 3 is a schematic diagram showing a tubular graphitic material comprising AA′ stacked graphene helices;

FIGS. 4(A)-(D) are schematic diagrams showing processes for preparing graphene ribbons according to an embodiment of the present invention;

FIG. 5 is a planar view of AA′ graphite showing the feature of the AA′ stacking of graphene layers;

FIG. 6(A) is a planar view showing the crystal structure of AA′ graphite, and FIG. 6(B) is a schematic diagram showing a space group of the AA′ crystal;

FIG. 7 is a planar view of AA graphite showing the feature of the AA stacking of graphene layers;

FIG. 8 is XRD patterns for samples with milling. Characteristic (002), (100), (004), (011) and (200) peaks for AA′ graphite were gradually broadened with milling. The arrowed peaks could be assigned to metal impurities originated from the steel balls; and

FIGS. 9(A)-(C) shows transmission electron microscope (TEM) images for pristine materials (A) and the samples milled for 1 hour (B) and 2 hours (C). The tubular AA′ stacked graphitic material (A) was totally destroyed by a two hour (Spex) milling (C), through to the graphitic ribbons where thickness is ˜5 nm (B).

DETAILED DESCRIPTION OF THE INVENTION

Description will now be given in detail of the present invention, with reference to the accompanying drawings.

A method for preparing graphene ribbons according to the present invention comprises (1) preparing graphite composed of helically stacked graphene ribbons, (2) cutting the graphitic material into a short form by applying energy to the graphitic material and (3) simultaneously with or followed by, decomposing an interlayer bond force thereby splitting the graphitic material into short graphene ribbon.

Hereinafter, the respective steps will be explained in more detail with reference to the attached drawings.

Preparation of the Graphitic Material

Graphitic material 1 according to the present invention has a structure that graphene ribbons 2 have helically grown along a long axis (refer to FIG. 3 and FIG. 4(A)). Here, the graphitic material 1 has a structure that at least two long-ribbons stacked.

Referring to FIG. 3, the graphitic material 1 is composed of helically grown long-ribbon shaped graphene formed by dislocation 3. The graphitic material 1 has a high aspect ratio more than 10, a diameter of several˜ several hundreds of nm (e.g., 2˜300 nm) and a length of several μm. The graphene ribbons 2 constituting the graphitic material have a width of—several tens of nm (generally, less than about ¼ of the diameters of the raw material, or ½ of the diameters of the graphitic material when it does not have a complete tubular shape), and have a length corresponding to that of the graphitic material.

The graphitic material may have a tubular or a fibrous shape. However, the present invention is not limited to those shapes, but can be implemented only that the graphene ribbon stacked body helically grows along the long axis.

The stacking type of graphene ribbons in the graphitic material may have a turbostratic (refer to FIG. 2) or an AA′. The turbostratic structure indicates the disordered stacking of graphene (i.e., there is no regularity in stacking between graphene layers). And, as shown in FIGS. 3 and 5, the AA′ stacked structure is a structure that alternate graphene layers exhibiting the AA′ stacking are translated by a half hexagon (1.23 Å).

The AA′ stacked structure is comparable with AB stacked structure (AB stacked graphite) known as the only crystalline graphite, and an AA stacked structure (AA stacked graphite) that can not energetically exist in nature but can be formed by intercalation of Li between graphene layers.

AB stacked graphite is described by a space group of a hexagonal (#194). Here, a=b=2.46 Å, c=6.70 Å, α=β=90° and γ=120°. That is, an interplanar spacing of the AB graphite is 3.35 Å A i.e., ½ of c.

AA stacked graphite is described by a space group of a simple hexagonal (#191). Here, a=b=2.46 Å, c=3.55 Å, α=β=90° and γ=120° (refer to FIG. 2). That is, an interplanar spacing of AA stacked graphite is 3.55 Å.

The structure of AA′ stacked graphite of the present invention could not be described with all of the 230 crystal space groups. Thus, we assigned the crystal structure of AA′ graphite to a simple hexagonal space group. Four atoms, consisting of two atoms on each of the A and A′ layers, are contained within the primitive unit cell of AA′ graphite. The former two atoms at (⅓, ⅔, ½), (⅔, ⅓, ½) are linked to the 2(d) site (⅓, ⅔, ½) of the space group whereas the latter two atoms at (⅙, ⅚, 0), (⅚, ⅙, 0) cannot be defined in the space group. Two kinds of both the (100) and the (110) planes appear, and we designate the distinctive planes as (100)* and (100)*, respectively. Due to a lack of experimental data concerning the atomic positions within the space group the X-ray diffraction (XRD) pattern of AA′ graphite was derived from that of AA graphite and it can be also derived from other space groups, particularly orthorhombic or monoclinic space group. The (001), (100), (102), (002), (014), (110), (112), (006), (200) and (022) peaks appear in the pattern of AA graphite. The (h01), (0k1) and (hk1) reflections are absent in AA′ graphite, due to the insertion of additional atoms from the A′ graphene layers into the eclipsed AA form. As a result the available reflections for AA′ graphite are due to the (002), (100), (004), (110), (006) and (200) planes, where the intensity of the (110) plane, that is (110)*, should be stronger due to the periodic overlap of graphene layers, as shown in FIG. 6A ((006) (2θ=84.4°) and (200) (2θ=92.6°) peaks are normally not observed because their intensities are too weak). One outstanding feature of the pattern of AA′ graphite is the disappearance of the (101) peak (2θ=44.6°), the (102) peak (2θ=50.4°) and the (112) peak (2θ=83.4°); the intensities are relatively strong within the pattern of AB graphite. Thus, the absence of the (101), (102) and (112) peaks within the XRD patterns of graphitic materials is a criterion for AA′ graphite.

The graphitic material comprising graphene ribbons of the present invention is generally obtainable with CVD (chemical vapour deposition) processes, using hydrocarbon gases such as C₂H₂, C₂H₄, CH₄ as a source of carbon under a vacuum state (below 760 Torr). Deposition temperatures are normally lower than 1000° C. Particularly, plasma assisted CVD processes can synthesize the graphitic material even at a low temperature of 500˜700° C.

Preparation of Graphene Ribbons

The graphitic material comprising graphene ribbons prepared in the first stage is decomposed into short graphene ribbons by applying energy to the graphitic material (refer to FIGS. 4(A)-(D)). For instance, mechanical cutting of the graphitic material having a large aspect ratio into a length less than a predetermined length (about several hundreds nm) can decompose it into short graphene ribbons 2 because the binding energy between graphene layers (Van der Waals bond) is weak. This is the same principle that straw bundles are decomposed into straws when the straw bundles are cut into a short length.

Methods for cutting the graphitic material may include a mechanical method (ball milling), a chemical method, and an electrical method (ionic milling utilizing plasma). As the mechanical method of the present invention, may be used a two-roller milling method, a ball milling method, an ultra high pressure spraying method, etc.

Mechanical ball milling is an easy method for fabricating graphene ribbons from a tubular graphitic material comprising AA′ stacked graphene ribbons (similar to conventional multi-walled carbon nanotubes (MW CNTS)). Milling time to decompose the material into graphene ribbons depends on milling energy. For example, a spex milling apparatus, which is known as efficient milling equipment, may completely decompose the graphitic material into short graphene ribbons within several hours. However, the graphitic material may not be decomposed by a longer milling even up to 100 hours if we use a milling apparatus with a small milling energy.

In the case of using tubular graphite as the pristine material, a process for crumbling the graphitic tube inducing a stress (stress crumbling) can be further included. The stress crumbling process is performed by penetrating water into the tubular graphitic material and freezing the water containing material. While the water is frozen, a tensile stress occurs in the tube due to a volume expansion. And, the tensile stress destroys the material into graphene ribbons (or powder). Here, an additional treatment for the tubular material to alter its hydrophobic characteristic to hydrophilic characteristic can be required.

Preferably, a sonication process after the crumbling processes (by the ball milling or the stress crumbling) can be added to completely scatter the crumbled graphene ribbons in liquid phase (refer to FIG. 4(C)).

Preferred Embodiment 1

Graphene ribbons were prepared by using a graphitic nanomaterial that graphene helices are stacked in an AA′ manner (similar to MW CNTs). Here, the graphite nano material has an average outer diameter of 20 nm (outer diameter distribution: 2˜50 nm), an average inner diameter of 3˜5 nm (inner diameter distribution: 1˜10 nm), and a length of 2˜3 μm. The sample was passed through a two-roller mill 50 times. This shortened it into short material ˜200 nm in length. Then, the processed sample was made to undergo a hydrophilic treatment, and then was immersed into water to penetrate water into the tube. Then, the short and water containing tubules were maintained at a temperature ˜10° C. for one hour, and then were melted. After a sonication (in alcohol) for 10 minutes, obtained were graphene ribbons having a width of about ˜5 nm and a length of about ˜200 nm (thickness of about 4 Å).

Preferred Embodiment 2

The same tube-type of graphitic nano material as that of the preferred embodiment 1 was passed through a two-roller mill 100 times, thereby having a length decreased into about 100 nm or less. Then, the sample was made to undergo a sonication process to be dried, obtained were graphene ribbons having a width of about ˜5 nm and a length of about ˜100 nm.

Preferred Embodiment 3

The same graphite nano material as that of the preferred embodiment 1 was milled for two hours using a spex ball milling apparatus. As an observation result for the milled sample by using a scanning electron microscopy (SEM), tubular materials were not observed. And, as an X-ray analysis result, the characteristic peaks of (002), (100), (004), and (110) of the AA′ stacked crystal gradually disappeared as the milling time increased (refer to FIG. 8). This means that the tube-type of AA′ graphene stacked body has been decomposed into graphene ribbons (C) through to stacked graphene ribbons (B) with the milling time as shown in FIGS. 9(A)-(C). For one hour milling graphitic ribbons coexist with bi- or single-layer graphene (B). With a further one hour milling, the graphitic ribbons were converted to graphene nanoribbons which are approximately 10 nanometres in length (C). Stacked graphene fringes are partially observed. Their average interplanar distance was measured to be about 3.55 Å (C). This supports the analysis that the graphene nanoribbons are stacked in a disordered arrangement i.e., commonly named turbostratic stacking.

Preferred Embodiment 4

Graphene ribbons were prepared by using carbon nano fiber composed of helical graphene (average diameter of 500 nm, and length of about 10 μm). The sample underwent a milling process for two hours. As SEM and X-ray analysis results of the sample, the same results as those of the preferred embodiment 2 were obtained. This shows that carbon nano fiber can be also decomposed into graphene by a milling process like the multi-walled carbon nanotubes.

Preferred Embodiment 5

The same tubular graphitic nanomaterial as that of the preferred embodiment 1 was prepared. To decompose the sample into graphene ribbons by an electric (plasma) energy, it was irradiated by a 200 W argon plasma for 10 minutes. The plasma was generated in a pressure of 50 mTorr. As an Atomic Force Microscopy (AFM) analysis revealed decomposed graphene ribbons where a width and a length are 2-6 nm and 5-50 nm, respectively (thickness: 0.4-1 nm).

The foregoing embodiments and advantages are merely exemplary and are not to be construed as limiting the present disclosure. The present teachings can be readily applied to other types of apparatuses. This description is intended to be illustrative, and not to limit the scope of the claims. Many alternatives, modifications, and variations will be apparent to those skilled in the art. The features, structures, methods, and other characteristics of the exemplary embodiments described herein may be combined in various ways to obtain additional and/or alternative exemplary embodiments.

As the present features may be embodied in several forms without departing from the characteristics thereof, it should also be understood that the above-described embodiments are not limited by any of the details of the foregoing description, unless otherwise specified, but rather should be construed broadly within its scope as defined in the appended claims, and therefore all changes and modifications that fall within the metes and bounds of the claims, or equivalents of such metes and bounds are therefore intended to be embraced by the appended claims.

Claims

1. A method for preparing graphene ribbons, comprising:

preparing a graphitic material where the graphitic material is composed of graphene helices or helical graphene ribbons grown along its long axis; and

shortening the graphitic material by applying an energy, simultaneously with or followed by, decomposing the graphitic material into graphene ribbons.

2. The method of claim 1, wherein the graphene ribbon stacked material is a turbostratic stacked structure or an AA′ stacked structure, and

wherein the AA′ stacked structure is a structure that alternate graphene layers exhibiting the AA′ stacking are translated by a half hexagon (1.23 Å).

3. The method of claim 1, wherein the graphitic material has an aspect ratio more than 10, and has an outer diameter of 2˜300 nm, and the graphene ribbons have a width equal to or less than ½ of the outer diameter of the graphitic material.

4. The method of claim 1, wherein the graphene helices are formed by dislocation.

5. The method of claim 1, wherein the graphitic material is a tube shape or a fibrous shape.

6. The method of claim 1, wherein the graphitic material is obtained by thermally-processing non-crystalline carbon material under an inactive atmosphere at a temperature of 1,000˜2,000° C., or

performing a chemical vapor deposition (CVD) method under conditions that hydrocarbon is used as reaction gas, an inner pressure of a synthesis container is 100˜1,000 Torr, a temperature of the synthesis container is 600˜1,000° C., and a gas flow amount is 50˜200 sccm.

7. The method of claim 1, wherein the graphitic material is a tube shape, and further comprising:

performing a hydrophilic treatment to penetrate water into the tube, and cooling the water included tubular graphitic material to induce a tensile stress.

8. The method of claim 1, further comprising a sonication process to scatter the processed graphene ribbons.

9. The method of claim 8, wherein the graphitic material is graphite tube having a tube shape, and further comprising:

performing a hydrophilic treatment for the cut graphite tube before the sonication process, then penetrating water into the tube, and cooling the water, thereby unfolding the cut graphite tube by a tensile stress occurring due to a volume expansion while the water freezes.